The invention relates to a hydrodynamic coupling and especially a compressed-air system for motor vehicles with such a hydrodynamic coupling.
Hydrodynamic couplings are known in a large number of configurations and in various applications, especially in the field of motor vehicles. It was proposed recently for example that a hydrodynamic coupling be provided in the drive train between an automotive engine, which is usually an internal combustion engine, and an air compressor of the compressed-air system of the motor vehicle in order to switch the compressor on and off via the hydrodynamic coupling. According to a special embodiment, the present invention relates to just such a compressed-air system for motor vehicles, especially for motor vehicles such as trucks.
Such a compressor, which is also known as an air compressor, in a vehicle is designed for a specific operating speed or for a speed range beneath the maximum speed which at least should not be exceeded permanently. 3000 revolutions per minute are permanently permitted for such a compressor for example, with the compressor being allowed to briefly also reach 3500 revolutions per minute. This relates especially to reciprocating piston air compressors. The coupling in accordance with the invention shall be configured for such a compressor for example.
In practice, it has proven to be advisable with respect to overall size and efficiency that the drive speed of the coupling is increased by adjusting the transmission when using a hydrodynamic coupling in the drive train between automotive engine and compressor. This can lead to operational states in which the pump side reaches up to 4500 revolutions per minute in air conveying operation. In the case of normal slip in the coupling, the turbine side which usually revolves at the same speed as the input shaft of the compressor will accordingly also exceed the said maximum permissible speed. Measures are therefore necessary in order to protect the compressor from respective excessive speed.
Although it is principally known to control the speed of a work machine by means of a hydrodynamic coupling by setting the degree of filling of the hydrodynamic coupling which is interposed between the engine and the work machine (see for example the published patent specification WO 98/32987). The control of the filling occurs substantially in such a way that the supply of working medium is interrupted upon reaching a maximum degree of filling of the working chamber.
The known feedback control for the filling quantity is complex with respect to the instrumental configuration and requires various feedback control components such as pressure sensors, valves and/or scooping pipes. Especially when a hydrodynamic coupling is used in a vehicle advantageously in the drive train between the air compressor and the drive engine in order to switch the air compressor on and off and in order to dampen vibrations and to avoid the transmission of torque from the air compressor to the drive engine or the transmission of the drive engine, the known configuration of filling-controlled hydrodynamic clutches is too complex and not ideally suited for speed monitoring of the work machine, especially the air compressor.
Document U.S. Pat. No. 2,873,831 A describes a hydrodynamic coupling with a centrifugal valve which revolves with the speed of the pump wheel of the hydrodynamic coupling. It can thus be prevented despite the rising speed of the engine that the speed of the work machine will rise further.
Documents FR 1 016 938 A and GB 2 227 078 A also describe hydrodynamic couplings with valves for discharging the working chamber.
Published patent specification DE 32 17 465 A1 proposes for discharging the hydrodynamic coupling in order to stop the turbine wheel that several discharge openings are arranged in the housing of the coupling which are controlled by valves. These valves are closed by a control pressure in lines for filling the coupling leading to the valves and open for discharging when the control pressure does not occur due to the centrifugal force acting upon their valve body. The valve bodies are therefore pressurized on the radial inside by the working medium from the working chamber of the coupling and on their radial outside also by the working medium which when the coupling is switched on is guided into the respective lines when the coupling fills up. A self-switching closed-loop speed control of the hydrodynamic coupling is not possible by such valves because for opening the valves the control pressure which acts radially from the outside on the valve bodies always needs to be degraded by means of an external circuit in such a way that working medium is supplied to the hydrodynamic coupling and thus to the working chamber. This embodiment is therefore unsuitable for limiting the speed of a work machine.
The features known from document DE 32 17 465 A1 are summarized in the preamble of claim 1.